1. Field of the Invention
The present invention relates to a car air-conditioner having a variable displacement refrigerant compressor, and more particularly, to a control apparatus for controlling the displacement of a continuously variable displacement type refrigerant compressor of the car air-conditioner.
2. Description of the Related Art
In a car air-conditioner with a variable displacement type refrigerant compressor, control of the displacement of the compressor, especially a continuously variable displacement wobble plate type refrigerant compressor, in accordance with a change in a refrigerating load in the air-conditioning circuit, in which one or more pressure responsive control valves accommodated in the compressor are used for automatically controlling the displacement changing mechanism of the compressor is known (e.g., U.S. Pat. No. 4,702,677).
Another car air-conditioner employing a variable displacement wobble plate type compressor having a solenoid-operated displacement changing unit (U.S. Pat. No. 4,747,754) is known in which a control unit including an electronic microcomputer or a microprocessing unit (CPU), and temperature sensors for sensing a temperature of an air discharged from an evaporator in the circuit and an actual temperature in a car compartment to be air-conditioned, are employed for electrically controlling the operation of a solenoid-operated displacement changing unit in the variable displacement refrigerant compressor. In this system, the CPU controls the solenoid-operated displacement changing unit of the compressor to control a PID motion, on the basis of the temperature detected by the temperature sensors to thereby obtain a desired temperature of the air discharged from the evaporator or a desired temperature of the car compartment.
During the control of the operation of the solenoid-operated displacement changing unit of the variable displacement wobble plate type compressor by the PID controlling motion, however, often a large oscillatory change occurs in a temperature of the air discharged from the evaporator or a temperature in the car compartment, to a point both higher and lower than the desired temperature, i.e., hunting occurs to cause a change in the controlled temperature. Accordingly, a satisfactory air-conditioning is not obtained, and the driver and passengers of the car are made uncomfortable.